Chemical
Equilibrium Continued: Figuring out
the equilibrium composition of our reaction mixture is only one part
of the problem. The second, and more interesting problem is
how to influence the reaction to move one way or another down the
reaction coordinate, presumably to increase the quantity of either
reactants or products. Today we talked about three effects on
z(eq).

Pressure: Kp is a constant that is not
dependent on total pressure of the system. If the total
pressure of the system changes, then the extent of reaction, z(eq)
must change to compensate, not Kp. By separating Kp
into expressions for mole fraction of each species (which we called
Kx) and Ptot, we can quantify how much z(eq)
must change in order to keep Kp constant.

Temperature: Kp is a function of temperature,
and so as temperature changes the value of the equilibrium constant
changes, and thus z(eq) changes. Using the
Gibbs-Helmholtz equation (which we derived), we derived an
expression for change in Kp caused by changes in
temperature through delta(Hrxn). By doing a careful
accounting of signs, we find that for an endothermic reaction, Kp
increases as Kp increases, as expected.

Composition: The extent of reaction will also be
influenced by changing the composition of the reaction
mixture. Again, we can use Le Chatlier to predict what will
happen if a reactant or product is added to the reaction
solution. In order to quantify this, we introduced a new
constant, Q, which is defined in the same way to Kp
at all points on the reaction coordinate except at
equilibrium. By comparing Q to Kp, we can
determine which direction the reaction will proceed spontaneously,
and how far it will go before achieving equilibrium.